The present invention relates to the production of microorganisms.
The production of microorganisms is performed within a growth medium incorporating nutrient substrata diluted in water. This growth medium is initially seeded with microorganisms, which thus develop within a quasi-closed environment, except for the supply of neutralizing agent (so-called "batch" culture). This kind of fermentation applied on an industrial scale offers the advantage of an uncomplicated nature at the cost of low efficiency, since restrictive factors apply, be it within the nutrient substratum itself or in the inhibiting agents generated by the microorganisms, the ever rising concentration of which inhibiting agents during fermentation has the result of reducing the growth rate of the microorganisms.
To eliminate his disadvantage, it has been proposed to provide a continuous fermentation, that is, to make provision for metered extraction from the growth medium simultaneously with a complementarily metered infeed of nutrient substratum, neutralizer and water. This procedure renders it possible to increase the production of microorganisms for a fermenter of given volumetric capacity, but this at the cost of a reduced microorganism concentration in the extracted material. This kind of fermentation (referred to as "feed batch"), which renders it possible to diminish the deleterious action of the inhibitors by keeping their concentration at a restricted level, nevertheless has the major disadvantage of requiring a fermenter of greatly over-dimensioned capacity, so that this solution has very rarely been adopted by the industry.
It has also been proposed to make use of dialysis membranes or diaphragms which, under certain conditions, render it possible to eliminate the inhibiting agents formed and thereby to raise the final concentrations of microorganisms in the growth medium. This solution has not been adopted by the industry because membranes of this nature are decidedly too weak mechanically, thermally and chemically; their operational control is chancy and complex, their mode of operation at variable volume is not very desirable, and the ponderal yield of microorganisms produced as compared to the amount of substratum fed into the medium remains low.
The ultrafiltration diaphragm technique rendered it possible to take a leap forward in the fermentation of biological products. A recycled flow from the said growth medium is forced through an ultrafiltration cell, which may be selected to permit extraction of the inhibiting agent or agents, whereas the microorganisms are kept within the flow recycled into the growth medium. However, a portion of the nutrient substrata is unavoidably withdrawn with these inhibiting agents. It is thus possible to raise the final concentration of microorganisms in the growth medium to two or three times that of the so-called "batch" process. However, despite its remarkable qualities, it has been observed that in this last process a very pronounced reduction occurred in the subsequent growth rate of the microorganisms after a high initial development.
It is a main object of the present invention to provide a process which increases the final concentration of microorganisms in a process of this nature, whilst retaining an acceptable ponderal yield relative to the consumption of nutrient substrata.